Chemical vapor deposition (CVD) systems normally employ a chamber in which gaseous chemicals react. From these reactions, a substance is deposited on a wafer surface to form dielectric, conductor, and semiconductor film layers that constitute an integrated circuit, for example. In a chemical vapor deposition system, a process gas is injected into the plasma chamber in which a plasma is formed. Due to the ion bombardment within the plasma of the process gas, (SiH.sub.4 (silane), for example) silicon will be deposited on a wafer which has been previously placed in the chamber. During this deposition step, the gas injection ports, also known as jet screws, typically clog with silicon-rich oxide residue formed by the combined SiH.sub.4 (the process gas) and oxygen radicals flowing into the gas injection port. These oxygen radicals originate from the plasma chamber.
The residue coats the walls of the chamber, and also tends to clog the gas injection ports. The chamber, as well as the gas injection ports, needs to be cleaned periodically. This ensures that each wafer encounters the same environment so that the deposition process is repeatable. Since opening up the chamber (changing out the hardware) for cleaning is very labor intensive and costly, a method for removing the deposition from the chamber walls without opening the chamber itself has been previously developed. This "insitu" cleaning has been accomplished in the past using fluorine. The fluorine is injected into the chamber as NF.sub.3. Fluorine is known to etch silicon and silicon dioxide at high rates when it is accompanied by ion bombardment. Radio frequency (RF) power provides the energy for ion bombardment, with the NF.sub.3 serving as the source of fluorine.
Typically, after a wafer is processed through deposition in the CVD system, the wafer is removed to a load lock. A cover wafer is then transferred to the chamber and placed on the chuck. The cover wafer is a standard silicon wafer that is coated with aluminum. It protects the chuck surface from the plasma cleaning and conditioning steps that follow.
The RF power is applied to the chamber and NF.sub.3 is injected into the chamber. The walls will then be cleaned of oxide deposition. However, there may still be a significant amount of fluorine in the chamber and on the walls and free particles. For this reason, a pre-deposition conditioning step is often required. The conditioning step is essentially a deposition that getters the fluorine and tacks down particles onto the chamber walls. When this pre-deposition conditioning step is completed, the cover wafer is transported back to its cassette and the next wafer can then be processed.
In conventional systems for routing the gas to the chamber, injection ports are shared between the deposition process gas (SiH.sub.4) and the insitu cleaning gas (NF3). Such an arrangement is shown in prior art FIG. 1 in which a portion of a process chamber is schematically depicted. The plasma chamber 10 injects oxygen at port 12 into the interior 14 of the plasma chamber. The oxygen radicals are formed within the plasma chamber 14. The shared injection ports for the deposition process gas and the insitu clean gas are depicted as reference numeral 16. During the deposition step, the gas injection ports (also known as "jet screws") clog with silicon-rich oxide residue formed by the combined SiH.sub.4 and the incoming oxygen radicals originating from the plasma chamber.
As stated earlier, the insitu cleaning gas is designed to chemically etch the SiO.sub.2 (silicon dioxide) residue. However, high pressure caused by supersonic gas flows in front of the jet screws causes regions of scarce fluorine radicals that reduce fluorine induced etching of the SiO.sub.2. FIG. 2 a schematic depiction of a detail of a jet screw. NF.sub.3 gas is injected into the chamber 14 through the jet screw 16. Within the jet screw, there is SiO.sub.2 clogging, schematically depicted at point 18 at the jet screw 16. The high pressure region 20 of scarce fluorine radicals caused by the supersonic gas flows in front of the jet screws 16 reduces the fluorine induced etching of the SiO.sub.2 in this area, and in particular, prevents the jet screws 16 from being unclogged of the SiO.sub.2 residue. All of the other chamber surfaces are typically cleaned except for the jet screw ports.
Due to the SiO.sub.2 clogging of the jet screw ports, the jet screws are normally replaced after approximately 300 wafers have been processed. This process involves shutting down the chamber at high expense and loss of productivity. Another problem of the prior art arrangement is that the SiH.sub.4 and NF.sub.3 gases, if combined, are highly combustible so that routing the gases through the same injection ports can be relatively dangerous.